DAB stands for Digital Audio Broadcasting and is a method for the terrestrial digital transmission of radio signals. DAB allows for a much more efficient use of frequency spectrum than traditional analog radio. Instead of just one service per frequency as is the case on FM, DAB permits up to nine (or more) services on a single frequency.
Multipath propagation interference that commonly disturbs analog reception, is caused by radio signals bouncing off buildings and hills, and is eliminated with DAB signals. Since DAB automatically selects the strongest regional transmitter, reception is much clearer.
Immunity to fading and interference caused by multipath propagation is achieved without equalization by means of the OFDM modulation techniques.
OFDM modulation consists of 1,536 subcarriers that are transmitted in parallel. The useful part of the OFDM symbol period is 1 millisecond, which results in the OFDM subcarriers each having a bandwidth of 1 kHz due to the inverse relationship between these two parameters, and the overall OFDM channel bandwidth is 1,537 kHz. The OFDM guard interval is 246 microseconds, which means that the overall OFDM symbol duration is 1.246 milliseconds. The guard interval duration also determines the maximum separation between transmitters that are part of the same single-frequency network (SFN), which is approximately 50 miles.
OFDM allows the use of single-frequency networks (SFN), which means that a network of transmitters can provide coverage to a large area—up to the size of a country—where all transmitters use the same transmission frequency. Transmitters that are part of an SFN need to be very accurately synchronized with other transmitters in the network, which requires the transmitters to use very accurate clocks.
When a receiver receives a signal that has been transmitted from the different transmitters that are part of an SFN, the signals from the different transmitters will typically have different delays, but to OFDM they will appear to simply be different multipaths of the same signal. Reception difficulties can arise, however, when the relative delay of multipaths exceeds the OFDM guard interval duration.
While DAB is commonly used in parts of the world, it is a relatively new transmission method. Coverage is still limited, and availability of the appropriate receivers is limited as well.
In order to provide complete coverage, it is a common procedure to simultaneously transmit or simulcast program material using both DAB and analog Frequency Modulated (FM) signals. DAB receivers are usually also capable of receiving both DAB and FM transmissions.
Since DAB receivers are commonly used in automobiles or other moving applications, there is a need to be able to seamlessly switch between the two transmission modes as the receiver moves between different transmission areas. The audio degradation modes of the two transmission modes is also different, so it is beneficial for the receiver to be able to select the transmission that has the best audio quality at any given time.
There are multiple methods known in the prior art to accomplish this goal. The following examples illustrate some of the known methods.    A) Simple switching—a decision is made in the receiver that determines which signal has a better quality, and that is selected by a simple transfer switch. This method may result in gaps in the audio due to the misalignment of the signals.    B) Simple blending—a decision is made in the receiver that determines which signal has a better quality, and the signals are mixed and ramped from one signal to the other without any time alignment. This may result in “confused” audio during the ramping due to time misalignment of the signals.    C) Sample correlation time alignment—a decision is made in the receiver that determines which signal has a better quality. After performing a sample by sample time alignment correlation, the signals are mixed and gain is ramped from one signal to the other. While this method will result in good audio quality, it is also very computationally intensive.